[Dubbo-issue1814][ Baiji-8] Add: xml conifg and ApplicationConfig for graceful-shutdown timeout #1814

[Heluwe]

What is the purpose of the change

Fix issue #1814

Brief changelog

dubbo-config-api/src/main/java/org/apache/dubbo/config/ApplicationConfig.java

Add variables Wait and their getter setter method.

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[htynkn]

1. the name should be wait or waitTime
2. I only see the new field in config, but where do you use this new field?